A constitutional lawyer who served in Ronald Reagan's administration says President Bush's "apparently criminal" authorization of a warrantless wiretapping program is grounds for the House to begin an impeachment inquiry.

By not beginning such an investigation, impeachment will become a "virtual dead letter," Bruce Fein, former deputy attorney general under Regan, said in a essay published in Slate.

Fein has emerged as a fierce critic of Bush's administration and has previously called for the president and Vice President Dick Cheney to be impeached.

"The House does not require, nor should it await, proof beyond a reasonable doubt of misconduct," Fein wrote in the essay published Friday. "To wait for such proof subverts the whole purpose of an impeachment inquiry."

Ample grounds for an impeachment inquiry lie in the president's shady justifications for his "Terrorist Surveillance Program," which Fein says subverted the check's on law enforcement powers inherent in the Constitution and Foreign Intelligence Surveillance Act.

"The president's apparently criminal spying continued until at least January 2007 -- or for more than five years [after 9/11] -- when Attorney General Alberto Gonzales declared that FISA warrants, whose nature remains classified, would replace the TSP," Fein writes. "(The attorney general maintained, however, that Bush continued to be crowned with Article II powers to ignore the warrant requirement and to do so secretly whenever he wished.)"

An impeachment inquiry should demand the president's legal justifications of the program, Fein writes, although he maintains that such an inquiry would not necessarily end with the president's ouster from office. He noted Abraham Lincoln's suspension of Habeas Corpus protections during the Civil War as an instance in which the national interest justified suspension of civil liberties.

Lincoln "rhetorically asked whether a republic must inescapably be too strong for the liberties of its people or too weak to maintain its existence," Fein writes. "The [House Judiciary] committee should similarly explore whether in the aftermath of 9/11, Bush was confronting such a dilemma."